When the power source for supplying power to drive a motor has stopped, it is necessary to urgently stop the motor for driving the feed shaft in a machine tool and the like. In such a case, in order to stop the motor in an emergency, there has been a known method whereby the speed command to be given to the motor control device is set to be zero so as to forcibly decelerate the motor at full torque (for example, Japanese Patent Application Laid-open H07-143780 (JP 7-143780 A).
However, there occur cases where regenerative energy from the motor for driving the feed shaft becomes excessive. When the regenerative energy becomes excessive in this way, use of the above conventional technology will cause the DC link voltage applied to the amplifier to rise due to the regenerative energy from the motor, hence the amplifier side will experience an overvoltage state. As a result, an overvoltage alarm arises and control of the motor may no longer be possible.
When an overvoltage alarm has arisen and control of the motor stops in the above way, the motor being in the course of deceleration starts to be decelerated by dynamic braking. Deceleration by dynamic braking will take a long time for deceleration. Therefore, the stopping distance of the feed shaft becomes longer, possibly giving rise to a problem that collision and damage take place in the machine in some cases. Herein, dynamic braking refers to a braking function of quickly stopping the motor by thermally consuming the rotational energy by forming a short-circuit between the terminals of the servo motor by way of a resistor.
The present invention is to provide a motor control device that can quickly stop a motor for driving a feed shaft by avoiding the occurrence of an excessive voltage alarm after a power outage.